1. Field of the Invention
The present invention relates to a variable focus lens and an optical device using this lens, and to a method of manufacturing a variable focus lens, by using an electrowetting phenomenon.
2. Description of the Related Art
Development of a variable focus lens device that uses the electrowetting phenomenon is disclosed in a Patent Document of Japanese Translation of PCT International Application No. 2001-519539 (hereinafter referred to Document 1, corresponding to United States Published Patent Application No. US2005/0002113A1), and in another document: S. Kuiper et al., “Variable-focus liquid lens for miniature cameras”, Applied Physics Letters, Vol. 85, No. 7, 16 Aug. 2004, pp. 1128-1130 (hereinafter referred to Document 2).
A variable focus lens described in each of the above-described documents is explained with reference to a schematic sectional constitutional view of FIG. 1.
This variable focus lens has a basic structure in which a container 10 of a cylindrical shape or the like is filled with a light-transmissible, conductive liquid material portion 1 and a light-transmissible, insulative liquid material portion 2 that does not mix with the above and that has a different refractive index and the same density (specific gravity). A first electrode 21 is formed to the outside from an internal wall of the container, covering one end portion thereof, and a dielectric film 32 is formed inside thereof. The end portion of the container 10 on the side covered by the first electrode 21 is fixed liquid tightly (a hermetically-sealed state not to leak liquid) with a light-transmissible material portion 11 made of glass, light-transmissible resin or the like, and a so-called bottom surface formed by this light-transmissible material portion 11 and the dielectric film 32 on the internal wall of the container 10 are covered and coated with a water-repellent material portion 23 called a water-repellent coat.
Then, on one end portion of the container 10 in contact with the conductive liquid material portion 1, namely on a lid-side portion, for example, a ring-shaped second electrode 24 is provided, and a light-transmissible material portion 12 made of glass, light-transmissible resin or the like is fixed liquid tightly (a hermetically-sealed state not to leak liquid) through a hydrophilic material portion 25 called hydrophilic coat. The edge portion of the ring-shaped second electrode 24 described above is extended and formed to cover the outer circumferential surface of this light-transmissible material portion 12.
Further, regarding the first electrode 21, an end portion on the internal surface side of the container 10 is separated from one end portion of the container 10, namely one end portion on the side where the second electrode 24 is arranged.
With the above structure, on applying a suitable voltage to the first electrode 21 and the second electrode 24 by voltage applying means 40a, the curvature of an interface between the conductive liquid material portion 1 and the insulative liquid material portion 2 is changed. By means of this, the lens action for light entered as shown with an arrow Li from the outside of the light-transmissible material portion 11 is changed, and a focal distance can be changed.
Explanation will be given about this, at first, when voltage is not applied to the first and second electrodes 21 and 24, an interface between the conductive liquid material portion 1 and insulative liquid material portion 2 becomes a part of a spherical surface of a certain radius, as shown in FIG. 1, by the balance of the surface tensions of the liquid material portions 1 and 2 filled and the internal surface of the container 10. For example, when salt water is used as the conductive liquid material portion 1 and silicon oil is used as the insulative liquid material portion 2, the interface becomes such that the water (conductive liquid material portion 1) has a convex shape, as shown in FIG. 1.
Then, when voltage is applied between the first and second electrodes 21 and 24, as if “wettability” of the conductive liquid material portion 1 were improved (this phenomenon is called electrowetting) on the internal wall surface of the container 10, and interface between the respective liquid material portions 1 and 2 changes such that a curvature radius of the interface becomes large, as shown in FIG. 2, for example.
Then, when light enters as shown with an arrow Li from a lid on one side of the container 10, namely from the light-transmissible material portion 11 and the light is emitted from the other lid, namely from the light-transmissible material portion 12, with a lens effect being caused by the difference in the refractive index and the curvature of interface between the liquids 1 and 2, and with the curvature of interface of the liquids being changed because of the electrowetting when voltage is applied, the focal distance is changed.
For example, as shown in FIG. 2A, a contact angle between the conductive material portion 1 and the dielectric film 32 is θ0 when voltage is not applied; on the other hand, as shown in FIG. 2B, when voltage V is applied, the contact angle θ(V) becomes small, and the curvature of the interface between both the liquids changes. In FIGS. 2A and 2B, the same numerals are given to the portions corresponding to those in FIG. 1 and the repetitive explanation thereof is omitted.